The invention concerns a method for automatically controlling the charge air temperature of an internal combustion engine.
The journal Schiff & Hafen/Kommandobrücke, No. 1/1990, pp. 49-50, describes a coolant circuit system of an internal combustion engine. The coolant circuit system consists of a high-temperature circuit with a coolant pump and a low-temperature circuit branching off from the high-temperature circuit. A thermostat valve, a recooler with a bypass line, a charge air cooler, and a lubricating oil heat exchanger are arranged in series in the low-temperature circuit. The coolant flow in the low-temperature circuit that branches off from the high-temperature circuit is distributed according to the position of the thermostat valve between a recooler coolant flow, which passes through the recooler, and a bypass coolant flow. After the recooler, the two coolant flows are brought back together and fed to the charge air cooler as the charge air cooler coolant flow. The thermostat valve thus determines, via the distribution of the coolant flow, the temperature of the charge air cooler coolant flow. The temperature of the charge air cooler coolant flow in turn defines, via the temperature difference versus the charge air temperature, the amount of heat extracted from the charge air in the charge air cooler. For example, when the internal combustion engine is at full load, the thermostat valve is completely open, so that the entire coolant flow of the low-temperature circuit flows through the recooler, and therefore the greatest possible amount of thermal energy is extracted from the charge air. When the internal combustion engine is idling, the thermostat valve is completely closed, so that the entire coolant flow of the low-temperature circuit flows through the bypass line, and very little thermal energy is extracted from the charge air. The switching state of the thermostat valve is determined by a thermostatic operating element such as an element made of a material which expands with temperature. When the temperature of the coolant flow increases, therefore, the thermostat valve expands and opens, and when the temperature of the coolant flow decreases, the valve contracts and closes with the assistance of spring tension. Because of the way in which the thermostat valve is designed, automatic control of the charge air temperature is still not possible at all operating points.
DE 201 22 420 U1 discloses an electrically heated thermostat valve in which the characteristic curve of the thermostatic operating element can be shifted by the electric control of the heating element. For example, when an internal combustion engine is cold and is required to operate under high load, the coolant flow can be influenced by the bypass line at an earlier time than would be possible by the thermostatic operating element. In the remainder of the text, a thermostat valve of this type will be referred to as a thermostat valve of the “characteristic diagram” type. DE 102 23 686 A1 discloses a corresponding method for controlling this characteristic-diagram type thermostat valve. It describes a two-position control arrangement with input control of the operating element. However, this method does not offer significant improvement with respect to the automatic control of the charge air temperature in the coolant circuit system described above.